Alvan Clark 5-in. Refracting Telescope

This telescope is one of the eight telescopes built for the US Naval Observatory's expedition to observe the transits of Venus in 1874 and 1882. Built by Alvan Clark and sons in Cambridge, Massachusetts, this particular telescope (no. 856) was used in Vladivostok, Siberia, in 1874, and in San Antonio, Texas, 1882. The telescope was loaned to the Panama Canal Zone government in 1928 where it was in use from 1930 until the late 1970's. It was returned July 1992, and was restored to present condition by the Astrometry Department Instrument Shop in the summer of 1996.
During a transit of Venus, the planet's dark disk appears to cross the bright face of the Sun. Through the 19th and 18th centuries the belief was strong that observations of the transits (a transit of Venus occurs four times in 243 years) could yield the precise Earth-Sun distance (Solar Parallax). Major nations sponsored expeditions to the most remote parts of the world to obtain the necessary data. Solar Parallax is now observed by other means. There are no transits of Venus in the 20th century; the next pair are not due until June 8, 2004, and June 6, 2012